The concentration of the ferment iron in living substance is very small, being in the region of 1 g to 10 million g of cellular substance.

Otto Heinrich Warburg

Green, red, and mixed shades of haemins are known. If magnesium is replaced by iron in chlorophyll, green haemins are obtained. Their colour is due to a strong band in the red which is already recognized in chlorophyll.

Otto Heinrich Warburg

The toxic effect of carbon monoxide on man has nothing to do with inhibition of cellular respiration by carbon monoxide but is based on the reaction of carbon monoxide with blood iron.

Otto Heinrich Warburg

Catalytic oxidation in living substances rests upon change of valency in an iron compound which is the respiratory oxygen-transferring ferment.